The present invention relates to a fixing unit which is used in a copying machine or a printer using toner to fix a toner image, and an image forming apparatus in which the fixing unit is incorporated.
A fixing unit is incorporated in a copying machine which uses an electrophotographic process, to heat and melt a developer or toner formed on a member to be fixed, for example, paper material (non-processed paper), to fix the toner to the piece of paper (paper material). As a well-known method of heating a toner usable in a fixing unit, there is a method of using radiant heat obtained by lighting a filament lamp or a halogen lamp, and a flash heating method using a flash lamp as a heating source. In recent years, a fixing unit which uses an induction heater as a heating source has also been put to practical use. A fixing unit using an induction heater can reduce the time required to heat a heating (fixing) roller or sheet up to a temperature for melting the toner.
In many cases, a heating roller containing a heater inside and a pressing roller pressed by a predetermined pressure to a heating roller or sheet at one point on its outer circumference, are used. In this structure, it is easy to efficiently supply the heat from a heat source to the toner, and to supply a pressure for fixing a melted toner to a piece of paper.
When an induction heater is used as a heat source, a current flowing in an excitation coil reaches several tens of amperes. Thus, when disconnecting the fixing unit or sliding it toward the front of a copying machine by a sliding mechanism to clear a paper jam, or when opening the cover or the like, the power supply to the induction heater must be interrupted. When the power supply to the induction heater cannot be interrupted by a structure, a cover or the like is separately provided to disable the user or operator from touching a current-carrying part.
However, if the unit is constructed to interrupt the power to the induction heater when removing or sliding the fixing unit or opening the cover for maintenance, for example, a drawer connector which can release electrical connection when being slid is used, and a problem of lowering a contact resistance through long-term use rises. This will cause an electrical short-circuit or other defects.
Further, provision of a cover to prevent the user or operator from touching a current-carrying part raises another problem that the shape and structure of the additional cover becomes complex. The additional cover requires an additional cost. If the current-carrying part is formed difficult to be touched by the user or operator, it will be difficult to clear a paper jam, or extremely decrease the workability in maintenance.